7.8 Namespace Association

Caution: The semantics of this extension are not fully
defined. Users should refrain from using this extension as its
semantics may change subtly over time. It is possible that this
extension will be removed in future versions of G++.

A using-directive with __attribute ((strong)) is stronger
than a normal using-directive in two ways:

Templates from the used namespace can be specialized and explicitly
instantiated as though they were members of the using namespace.

The using namespace is considered an associated namespace of all
templates in the used namespace for purposes of argument-dependent
name lookup.

The used namespace must be nested within the using namespace so that
normal unqualified lookup works properly.

This is useful for composing a namespace transparently from
implementation namespaces. For example: